1. Field of the Invention
The invention relates in general to distributed computer systems, and more specifically to new and improved methods and apparatus for correcting the software clock of a microcomputer located remotely from a central computer having an accurate clock.
2. Description of the Prior Art
It is common in distributed computer systems for a remote computer to collect data related to time maintained by the remote computer, with a central computer periodically reading and interpreting the data. For correct interpretation, it is usually important that the remote clock accurately reflects real time. The crystal controlling the remote computer's internal clock has an original error, requiring special initial testing to determine the number of crystal pulses generated in one second, and subsequent special programming of the value into the read-only-memory (ROM) of each remote computer unit. The internal clock of the remote computer, however, is also subject to additional errors, not initially determinable, such as errors due to temperature and aging. A conventional method for correcting such errors would be to simply down load the correct time each time the central computer interrogates the remote computer. This has the disadvantage of introducing a step input to the value of the remote time, and the error can progressively become worse as time and/or temperature effects cause the oscillator frequency to drift more and more from the design value. The step input causes data errors, such as elapsed time measurements. It can cause processes performed on a real time basis to be performed twice if the clock is set back behind the time for a process already performed, or it can cause processes to be skipped entirely if the clock is set ahead beyond the time for the process.